This invention relates to a method of joining first and second components, and is particularly, although not exclusively, concerned with a method of repairing a structure, such as a bladed disc of a gas turbine engine, by bonding a patch component to the damaged structure.
Compressors and turbines of gas turbine engines such as those used in aero engines typically include a plurality of rotor and stator vane assemblies. The rotor assemblies are designed to impart work into gases passing through the compressor and extract work from the gases passing through the turbine. The stator vane assemblies help to direct the working gases entering or exiting the rotor assemblies and thereby increase efficiency of the engine.
Each rotor assembly includes a disc and a plurality of blades attached to the disc so as to extend radially outwardly therefrom. Conventionally, the blades have been attached to the disc by mechanical connections such as “fir tree” type connections where a fir tree shaped blade root is received within a complementary shaped recess in the disc. This means that the blade can be readily replaced in the event that it is damaged.
Recent developments have resulted in integrally bladed rotor assemblies or “blisks” in which the blades are formed integrally with the disc. These have the advantage of reduced weight as compared to a standard rotor assembly and in improved aerodynamic efficiency. Such blisks are particularly applicable in the design of military aero engines.
In view of the nature of their applications, blisks are susceptible to damage. Blisks of gas turbine engines are high value structures. If a blisk is damaged during the manufacturing process, or during service, it is economically desirable for the damaged blisk to be repaired, rather than scrapped and replaced by an entirely new blisk.
Minor damage can often be rectified by conventional hand dressing and blending processes. However, more substantial damage, for example where more than 20% of an aerofoil volume is damaged or destroyed, repair can be difficult. In some circumstances, the damage may require the replacement of all or part of a blade. In order to replace the blade, it must be removed by machining it off to leave a stub, and fixing a new blade onto the stub by a suitable process.
It is known to remove a damaged aerofoil section and to effect a “patch” repair by joining a repair patch to the remaining part of the original aerofoil. If the joint is made by fusion welding, the resulting joint may have poor mechanical properties resulting from defects and discontinuities in the weld itself. Also, large heat affected zones are created in the repair patch and the original aerofoil part, in which the material properties may be altered.
Repairs have also been effected by using material deposition techniques such as Direct Laser Deposition or shaped metal deposition to regenerate the geometry of damaged components. Such processes are expensive and also result in a heat affected zone.
It has also been proposed, for example in U.S. Pat. No. 5,788,142, to join the metallic parts by means of an intermediate part made from a compatible powdered metal. The intermediate part is placed between the parts to be joined, and subjected to a reaction sintering and consolidation process to form the joint. Such a process requires the separate creation of the intermediate part, and the requirement for accurate alignment of both the parts to be joined and the intermediate part, which is difficult to achieve.